Many high performance, high solids coating compositions in the art are based on polymeric systems comprised of either polyester-based or polyacrylic-based polyols and crosslinking agents thereof. These coatings are generally supplied as two component or “two-pack” systems. In a typical two-pack system, the crosslinking agent is combined shortly before application, with curing being conducted at ambient or elevated temperatures. While two pack systems often provide high performance properties like corrosion and humidity resistance, resistance to solvents, ultraviolet stability and gloss retention there are notable limitations. Two-pack systems utilizing isocyanate crosslinkers require special handling and storage operations to prevent premature reaction with moisture and to avoid human exposure. Further, the components of two-pack systems can only be mixed shortly prior to use and once mixed must be used and ultimately be discarded. Another disadvantage of isocyanate-crosslinking systems is that the quality of the coating is compromised by bubble formation during thick film application. A more desirable coating composition is an ambient curing, one component or one-pack system where all of the coating ingredients are combined into a single, storage stable mixture. At the same time the one-pack system should exhibit the same high performance film properties as a two-pack system and be isocyanate free.
The coating composition of the present invention is a one-pack acrylic system that has the desired film properties of two-pack polyurethane coatings, such as corrosion and humidity resistance, short dry times, chemical resistance, good ultraviolet resistance and high gloss potential. The coating is moisture-curable, isocyanate-free, and VOC compliant, with a high solids content (>70% by weight) and a low VOC (<3 pounds per gallon). On exposure to moisture, the applied coating composition of the invention is a crosslinked reaction product of bis-silylamine and an acrylic polymer having pendant alkoxysilane groups. The acrylic polymer may also have pendant epoxy groups.
The prior art teaches moisture curable coatings formed from (meth)acrylate polymers containing pendant alkoxysilyl groups. These are described in U.S. Pat. Nos. 3,453,136; 3,453,230; 4,603,064; 4,614,777; 5,017,668; and 5,705,651. U.S. Pat. No. 5,399,607 discloses a mixture of acrylic resins with hydroxyl and alkoxysilyl containing acrylic copolymers to make moisture curable coatings.
U.S. Pat. No. 4,789,710 depicts silanated resins reacted with amino-silanes and further reacted with an isocyanate-ftunctional alkoxysilane. U.S. Pat. No. 5,017,668 describes a long chain amino-silane monomer copolymerized to make a curable acrylic silane polymer. U.S. Pat. No. 5,459,205 discloses a process to make more flexible coatings based on moisture curable resins having long pendant silane-containing moieties. These resins are prepared from silane containing macromonomers copolymerized with conventional monomers. The macromonomers may contain aminosilane moieties; however, the coatings require baking at 60° C. with a tin catalyst.
U.S. patent application Ser. No. 2002/0040102 describes an ambient temperature curing coating composition comprising a polysiloxane, an alkoxysilyl-functional acrylic polymer, and a curing catalyst. The invention further relates to the preparation of an alkoxysilyl-functional acrylic polymer.
We have found that coatings consisting of the silane-functional acrylic polymers and amines, as described in the above prior art, lack the desired solvent resistance of two-pack urethane coatings. Primary aminosilanes such as aminopropyltrimethoxysilane, when reacted with silane-functional acrylic polymers, offer some improved solvent resistance. However, according to this invention, it has been found that coatings consisting of the same silane-functional acrylic polymers, when reacted with secondary aminosilanes such as bis-silylamines, exhibit much improved solvent resistance rivaling that of two-pack urethanes.